Pressure relief valves are used in many industrial applications to prevent plant operating systems from reaching dangerously high pressures. Such valves include a nozzle assembly having a valve seat which is normally closed by a valve member slidably disposed in the body of the valve. The valve member typically is biased in the closed position against the valve seat by a compression spring, by fluid pressure, or both. With the valve in the closed position, when the pressure in the nozzle passage exceeds a predetermined set value, the valve member "pops" open and places the nozzle passage in fluid communication with an exhaust port in the valve body. When the pressure in the nozzle passage decreases by a specified incremental amount, termed the blowdown differential or simply "blowdown", to arrive at the reseating pressure for the valve, the valve member is again seated on the valve seat to close the valve.
Ideally, the set point (the pressure at which the valve opens), the blowdown and the reseating pressure should be controlled to close tolerances in order to ensure the safety of personnel and equipment in the vicinity of the high pressure plant operation. The blowdown pressure differential can be regulated by means of a secondary annular orifice defined by means of a valve member skirt surrounding an adjustment ring on the nozzle assembly. For a given set pressure, the blowdown can be increased by moving the adjustment ring upward in the direction of the valve member, thus decreasing the cross sectional area of the secondary annular orifice.
Another important consideration is seat tightness in order to minimize valve leakage, particularly at pressures approaching the set pressure of the valve. Seat tightness can be defined in terms of the relationship between the set pressure of the valve and the pressure, expressed as a percentage of the set pressure, at which the valve first begins to leak. Seat tightness can be improved by providing highly machined surfaces between the valve seat and the closure face of the valve member. In order to provide such a high quality valve surface, it is a conventional practice to face the valve member with a highly machined replaceable disc formed of stainless steel or like material. The disc is secured in the valve member or "disc holder" by means of a "disc retainer" e.g. a split ring secured in a groove in a disc stem and resting against internal shoulder within the disc holder.
As noted previously, valve leakage can be minimized by precision machining of the contacting surfaces of the valve seat and the valve member disc. Thus, the contacting surfaces of high quality valves are machined to a surface finish of less than three microinches in order to achieve positive valve seating. Valve leakage can also be reduced by employing valve seats of a relatively narrow width which offer a small contact area with the disc member. Valves having a narrow valve seat are more precise in their operating characteristics than valves having wider seats and can provide a high degree of seat tightness. However, the use of narrow seat valves is limited by the forces involved in valve opening and closing, particularly in a valves designed for high pressure operations. As a practical matter such high pressure valves, valves designed to operate at set pressures of 1000 psi or above, require valve seats having a width in excess of 0.8 mm., and it is not uncommon for such minimum seat widths to range up to 0.1 inch or above, i.e., well above 2.0 mm.
One technique for achieving seat tightness in high pressure valves involves the use of a so-called "soft seat" valve configuration. In this design, a resilient seal is provided by means of an O-ring secured in the disc holder by means of a disc shaped O-ring retainer. The conforming surfaces of the valve seat of the nozzle and the O-ring retainer are beveled with the valve seat projecting outwardly from the retainer periphery so that the metal to metal surfaces of the retainer and the valve seat bear the load imposed upon the disc holder. At the same time, contact between the outer edge of the valve seat in the O-ring provides the valve sealing function.
Pressure relief valves of the type described above are disclosed in Dresser Industries Inc.'s Catalog SRV-1 "Consolidated.RTM. Safety Relief Valves" (1984), Sections 1 and 2. Soft seat valves are particularly disclosed in Section 2, pages 13-15.